This video is sponsored by Masterworks. The image you are looking is that of the sun,
but here is the astounding part - it was not taken looking up at the sky, but taken looking DOWN at
the earth? How the heck is that possible? Well, this is a photo of the
neutrino emission of the sun. Neutrinos are so small and interact
so little with other particles, that nearly all of them pass right through the 12,700 km diameter of
the earth without hitting a single atom. The sun produces a mind-boggling 10^38
neutrinos every second. In fact, about 100 trillion neutrinos are passing
through your body every second right now. Don’t be alarmed though. They are
harmless. You can’t feel them whatsoever, because they are not interacting with anything.
They are kind of like a ghost that can pass right through walls.
In fact it’s estimated that a single neutrino could pass through a one light year thick chunk
of lead without hitting anything. So the question you might ask,
if they don’t interact with anything, what purpose do they serve?
Nothing is built from these particles. They are seemingly useless.
Yet they are everywhere, and in fact, you wouldn’t exist without them. How can this
be? What are these ghost particles? And what purpose do they serve in
the workings of the universe? That’s coming up right now…
I think you’re going to be fascinated by the story of Neutrinos, which you’ll find
are really responsible for everything. But before I get into that, I want to acknowledge
Masterworks for sponsoring this video. It’s been a tough year. Headlines are
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This is Goldman Sachs saying that real assets can help salvage lost
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Every one of their exits to date has delivered positive returns, Handing back over $25 million
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and the Masterworks SEC.gov page. Masterworks has 675,000-plus users,
and their art offerings have sold out in hours - But I’ve received priority access for all of you
at the link in the description. I just found Masterworks to be a
great way to diversify for myself. And I think it’s worth a look. And now back to Neutrinos.
Neutrinos are one of the most mysterious particles we know of. And although they may
appear to serve no useful function, they are in fact very important.
For one thing they are ubiquitous. They are the second most abundant particles we know of, after
photons. When a supernova explodes 98% of the energy is carried by neutrinos within the first
10 seconds, not photons as you might have thought. They do not carry a charge, and they interact very
weakly. This why they can go right through the earth without interacting with anything.
You might ask, if they go right through the earth, right through our bodies, then they
probably go right through our instruments, so how the heck did we even know they existed?
Well, they were first theorized in 1930 by none other than Wolfgang Pauli, who is perhaps most
well known for the Pauli Exclusion Principle. He was trying to solve a known conundrum in
physics at the time. The conundrum was that some of the energy in a radioactive Beta decay
appeared to be missing. A beta decay occurs when an atom has too many neutrons or protons in its
nucleus. When there are too many neutrons, the nucleus becomes more stable by transforming a
neutron into a proton. This is called a Beta minus decay. This process emits an electron.
Scientists had noticed that the electron which was emitted did not always have the same energy.
Sometimes it had more, and sometimes it had less. But since the proton always
had a fixed value for its mass, they couldn’t understand how the electron’s energy varied.
This appeared to violate a sacrosanct principle in physics, the principle of conservation of energy.
Pauli proposed that perhaps a second invisible particle was being emitted along with the
electron, which carried the different energies, allowing for overall energy to be conserved.
But if it existed, it became clear that it must be neutral in order to conserve charge, be very light
and weakly interacting because it was not easily detected. Enrico Fermi soon after coined the term
Neutrino for this, as yet undiscovered particle. Now the question you have to ask how were they
experimentally detected if they don’t interact with anything? Well what’s favorable for detection
is the fact that they are so extremely abundant, to the tune of 100 trillion going through your
body every second, it turns out that if you build a detector large enough, and leave it on for long
enough, a small percentage of these neutrinos will interact often enough that we can detect them.
In fact they were confirmed experimentally in 1956, 26 years after first being theorized,
by physicists Fred Reines and Clyde Cowan, for which a Nobel Prize was later awarded. Their
detector weighed 10 tons and was placed near a powerful fission reactor. The fission and fusion
processes produce a lot of neutrinos. The tremendous fusion reactions inside
stars like our sun is the reason these particles are so abundant.
How are they produced in the fusion process? When the sun fuses Hydrogen to Helium,
this is a multistep process, the end result of which turns 4 protons into a helium nucleus
consisting of 2 protons and 2 neutrons. This process produces two positrons and two neutrinos.
Now you might say, if that’s the case, we should be as bombarded with streams of positrons as we
are with neutrinos. Why doesn’t that happen? The reason is because neutrinos don’t interact with
anything and pretty much pass right through the sun and onto earth. The positrons on the
other hand, being the antiparticle of electrons, are immediately annihilated by electrons in the
plasma of the sun, to produce high energy photons, or gamma rays. But by the time they make it out of
sun’s core, we see them as mostly visible light. So we should be thankful that we are bombarded by
visible light instead of positrons or gamma rays. The neutrino emission from the sun is where
this particle gets a little weird, and the mystery of its mass comes about.
They were initially thought to be massless, but we now believe that they do have mass because the
neutrinos from the sun went missing. What do I mean by this?
We know how many neutrinos are supposed to be produced by nuclear reactions in the Sun. And
these can be detected here on the Earth. However, we didn’t detect nearly as many neutrinos as we
should have. This was called the “Solar Neutrino Problem.” So where did the missing neutrinos go?
Well, it turns out that neutrinos, like all fermions actually, come in three different types,
called “flavors.” The flavors are electron neutrinos, muon neutrinos and tau neutrinos.
Electron neutrinos were the ones we were looking for, but when we learned how to detect the other
two flavors, muon and tau neutrinos, suddenly the missing neutrinos could be accounted for. Somehow,
they changed from electron neutrinos into muon and tau on their way here from the Sun.
This changing of flavors can only happen if these three particles have a non-zero mass.
What do flavors have to do with their mass? Well, this is where it gets weird. I’ll
give you the simple answer, then I’ll give you the more complicated answer.
The simple answer is that in order for flavor change to happen, some passing of time must occur
for the particle. In other words, if no time passed for the particle, then no change could
happen. But since flavor change does occur, time must be passing for them. If time is passing for
them, then they must be traveling less than the speed of light, because at the speed of light no
time passes, as is the case for photons. Now, if they are traveling less than the speed of light,
this means they must have some mass, because physics dictates that if they had no mass,
they must travel at the speed of light. Now, here’s the simplified version of
the more complicated answer. Remember the uncertainty principle in quantum physics,
where a particle cannot have precise position and a precise momentum at the same time? That is,
when a particle has a well-defined position, it will not have a precise momentum. And when it has
a well-defined momentum, it doesn’t have a precise position. In other words, if we know know
how fast it’s going, we don't know where it is.
And if we know where it is, we don't know how fast it's going
Something similar appears to be going on with neutrino mass and flavor. When a neutrino has
a precise mass, it’s flavor is mixed. When it has a precise flavor, its mass is mixed.
So what appears to be happening is that on its way to earth,
the neutrino is in a kind of superposition or a mixed state of various possible masses and
flavors. This is called neutrino oscillation. Unlike the uncertainty principle there is no
reason why we should not be able to measure the mass of the neutrinos. In principle, the mass
could be measured, but they are just so light and noninteractive, that we don’t have a way to
pin down its masses precisely. At least not yet. The probability of it flipping, or “oscillating”
between, the electron and muon neutrino flavors can be precisely
calculated. And observations confirm this. The source of this neutrino mass, and hence
the oscillation is the mystery. We don’t know why they oscillate. And this needs to be explained.
But because this flipping or oscillation occurs, we know there must be some mass, even though
we don’t know the mechanism of this mass. We just know that there are three neutrino
flavors and three neutrino masses. We also know that the electron neutrino mass cannot be
more than about 0.8 eV, which is on the order of a millionth of the mass of the electron.
Now, you might say, why is its mass a mystery? Why can we explain it using the Higgs Field,
which is the source of mass for all the other massive elementary particles?
In order to understand why, I have to explain in a simplified way, how the Higgs Field confers mass.
The Higgs flips the handedness or “chirality" of fermions if it interacts with them.
So when we say that the mass of the electron is created by its interactions with the Higgs field,
we can think of this as the Higgs field rapidly changing a left-handed electron
into a right-handed electron, and vice versa. This switching back and forth is energy. And
because energy and mass are equivalent via E=mc2, this energy shows up as its rest mass. A heavier
particle like the top quark would experience this flipping at a much higher frequency than
a lighter particle like the electron. And that’s why it would have a higher mass.
This mechanism however doesn’t work for the neutrino because it stays in its
left-handed state forever. Right handed Neutrinos don’t exist, or at least have
never been detected them. So this is why they are thought not to interact with the Higgs.
So the source of the mass of the neutrino remains a mystery, and seems to point to new,
as yet undiscovered physics. And it confirms that the Standard Model is incomplete,
which is not a bad thing because this could be an exciting area of research for physicists.
Now, I stated in the intro that we wouldn’t exist
without neutrinos. How could I make such a strong statement.
For one thing although their mass is tiny, there are so many neutrinos in the universe
that their abundance results in them playing a very important role in the evolution of the
large-scale structures of the universe, such as the distribution of galaxies.
And remember the Beta plus decay I talked about inside the sun? Well without it,
fusion could not occur, so the sun wouldn’t shine
without Neutrinos. And no life would exist without the sun.
Furthermore, neutrinos are also involved in Beta Minus decay. This decay allows
free Neutrons to decay into protons. Without this process, the universe may have consisted
of mostly neutrons, so much fewer atoms may have formed, precluding formation of life.
So there is a lot of interest among physicists and astronomers for studying neutrinos because these
seemingly useless ghosts appear to be pointing to new physics. And this might allow us to have
a breakthrough in our understanding of the way the universe really works.
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